Ketoglutarate complexes

We detenuined the influence of oxy- and ketocarboxylic acids (succinate, fumarate, adipinate, a-ketoglutarate, isocitrate, tartrate, E-malate) on the luminescence intensity of the Eu-OxTc complex. These substances interact as polydentate ligands similarly to citrate with the formation of ternary complexes with Eu-OxTc. As to succinate, fumarate, adipinate and a-ketoglutarate this they cannot effectively coordinate with EiT+ and significant fluorescence enhancement was not observed. 

Lipoic acid is an acyl group carrier. It is found in pyruvate dehydrogenase zard a-ketoglutarate dehydrogenase, two multienzyme complexes involved in carbohydrate metabolism (Figure 18.34). Lipoie acid functions to couple acyl-group transfer and electron transfer during oxidation and decarboxylation of a-keto adds. 

AceCyl-CoA + oxaloacetate + HgO. CoASH + citrate 2. Citrate. isocitrate 3. Isocitrate + NAD. a-ketoglntarate + NADH + CO, + 4. a-Ketoglntarate + CoASH + NAD. snccinyl-CoA + NADH + CO, + H Citrate synthase Aconitase Isocitrate dehydrogenase u-Ketoglutarate dehydrogenase complex... 

Composition of the n-Ketoglutarate Dehydrogenase Complex from E. coli ... 

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of cr-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an -keto acid loses C02 and is oxidized to a thioester in a series of steps catalyzed by a multienzynie dehydrogenase complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate vlide, followed by decarboxylation, reaction with lipoamide, elimination of TPP vlide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

Several enzymes of the intermediary metabolism require thiaminpyrophosphate (TPP, Fig. 1) as coenzyme, e.g., enzymes of the pyruvate dehydrogenase complex, a-ketoglutarate dehydrogenase complex, or pentose phosphate pathway. 

Figure 7-4. Ping-pong mechanism for transamination. E—CHO and E—CHjNHj represent the enzyme-pyridoxal phosphate and enzyme-pyridoxamine complexes, respectively. (Ala, alanine Pyr, pyruvate KG, a-ketoglutarate Glu, glutamate).

Pyruvate and a-ketoglutarate dehydrogenase have complex systems involving lipoate and FAD prior to the passage of electrons to NAD, while electron trans-... 

Four of the B vitamins are essential in the citric acid cycle and therefore in energy-yielding metabolism (1) riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor in the a-ketoglutarate dehydrogenase complex and in succinate dehydrogenase (2) niacin, in the form of nicotinamide adenine dinucleotide (NAD),... 

Thiamin has a central role in energy-yielding metabo-hsm, and especially the metabohsm of carbohydrate (Figure 45-9). Thiamin diphosphate is the coenzyme for three multi-enzyme complexes that catalyze oxidative decarboxylation reactions pymvate dehydrogenase in carbohydrate metabolism a-ketoglutarate dehydro-... 

Diaz, F. and Komuniecki, R.W. (1996) Characterization of the alpha-ketoglutarate dehydrogenase complex from Fasciola hepatica potential implications for the role of calcium in the regulation of helminth mitochondrial metabolism. Molecular and Biochemical Parasitology 81,243—246. 

Another model is based on the fact that the genetic code shows a number of regularities, some of which have already been mentioned above. It is suspected that codons beginning with C, A or U code for amino acids which were formed from a-ketoacids (or a-ketoglutarate, 1-KG), oxalacetate (OAA) and pyruvate. This new model, which is quite different from the previous models, assumes that a covalent complex formed from two nucleotides acted as a catalyst for chemical reactions such as the reductive amination of a-ketoacids, pyruvate and OAA. More recent analyses suggest that the rTCA cycle (see Sect. 7.3) could have served as a source of simple a-ketoacids, including glyoxylate, pyruvate, OAA and a-KG. a-Ketoacids could, however, also have been formed via a reductive acetyl-CoA reaction pathway. The bases of the two nucleotides specify the amino acid synthesized and were retained until the modern three-letter codes were established (Copley et al., 2005). 

Stability constants for calcium complexes of a selection of hydroxycarboxylate ligands are listed in Table VII (239,246,272-274). For tartrate, malate, and citrate stabilities decrease in the expected order Ca2+> Ba2+> Ra2+ (231,275). The stability constant for the complex of pyruvate (logiOifi 0.8 (273)) is similar to that for acetate calcium complexes of a-ketoglutarate and of oxaloacetate are somewhat more stable (logio-Ki = 1.3, 1.6 respectively (273)). The sequence logio-Ki = 3.0, 1.4, 1.1, 0.6 for the dicarboxylate ligands oxalate, malonate, succinate,... 

This is not the only example of Nature inventing the assembly line a long time before Henry Ford—both pyru-vate dehydrogenase and a-ketoglutarate dehydrogenase, mentioned earlier in the chapter, are also multi-enzyme complexes. 

There are some very interesting questions of stereospecificity posed by the structure and mode of operation of multienzyme complexes. Reed and Cox 35> have summarized available information on the pyruvate and a-ketoglutarate dehydrogenase complexes, and the fatty add synthetase. The mechanism of synthesis of the peptide antibiotics likewise presents interesting stereochemical problems 36>. 

It has been shown that JHDMIA demethylates only mono- and dimethyl lysine of histone H3 (H3K36). This was rather unexpected given that the catalytic mechanism of a Jmj C domain does not require a protonated nitrogen, unlike LS D1, and therefore could also have the ability to remove the methyl in all three states. So far, two crystal structures have been deposited in the protein database but without any publication underlining the work. The crystal structures describe two forms of the human JHDMIA lysine demethylase, apo and in a complex with a-ketoglutarate (PDB codes 2yul and 2yu2). 

C. Conversion of a-ketoglutarate to succinyl CoA, COj, and NADH is catalyzed by the a-ketoglutarate dehydrogenase complex. 

By analogy to the PDH complex, the a-ketoglutarate dehydrogenase complex is made up of three enzyme activities with a similar array of activities and coenzyme requirements. 

Arsenic can react Irreversibly with the critical sulfhydryl groups of the coenzyme Tipolc acid, which Inactivates the coenzyme and thus Inhibits the PDH complex and the a-ketoglutarate dehydrogenase complex. 

The answer is A. This patient exhibits several signs of acute arsenic exposure, including the cholera-like gastrointestinal symptoms and probable dehydration. He may currently be in hypovolemic shock and beginning chelation therapy is the only recourse. Arsenic is a metabolic toxin because it inhibits enzymes that require lipoic acid as a coenzyme the PDH complex, the a-ketoglutarate dehydrogenase complex, and trans-ketolase of the pentose phosphate pathway. 

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